Directional microphone with frequency independent beamwidth



March 24, 1970 M. R. SCHROEDER .ETAL 3,502,811

DIRECTIONAL MICROPHONE WITH FREQUENCY INDEPENDENT BEAMWIDTH Filed Dec.11, 196'? FIG./

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A TTOR/VE V United States Patent 3,502,811 DIRECTIONAL MICROPHONE WITHFREQUENCY INDEPENDENT BEAMWIDTH Manfred R. Schroeder, Gillette, andRobert L. Wallace,

Jr., Warren, N.J., assignors to Bell Telephone Laboratories,Incorporated, Murray Hill and Berkeley Heights, N .J., a corporation ofNew York Filed Dec. 11, 1967, Ser. No. 689,373 Int. Cl. H04r 1/20, N34US. Cl. 179-1 12 Claims ABSTRACT OF THE DISCLOSURE Fall-off of beamwidthwith frequency in a directional microphone system employing a signalreceptor located at or near the focus of a reflector is overcome byusing, effectively, a number of transducers spaced along the axis of thereflector. The frequency range of the system is divided into a number ofsubbands and signals are developed individually for each. Sincedefocusing broadens the width of the acceptance beam, the individualpoints of reception are selectively spaced along the reflector axis toachieve sufficient defocusing to assure a constant beamwidth for allsubbands. All subband signals are added together to produce an outputsignal.

BACKGROUND OF THE INVENTION This invention relates to techniques forconcentrating sound for selective reception and, in particular, toimproved directional microphones arranged to exhibit appreciable gainover a broad frequency range of sounds emanating from within arelatively narrow acceptance beam.

Field of the invention Directional microphones find use in a variety ofservices, for example in broadcasting, in conference telephone systems,and, in general, wherever it is necessary to select certain receivedsounds from among others. Since the apparent intensity of receivedsounds is dependent upon the beamwidth of the microphone, it isgenerally desirable, for directional reception, that the beamwidth beextremely narrow, of the sort normally defined as a pencil beam.Although concentration of sound into such a narrow beam is relativelysimple at selected frequencies, the degree of concentration falls off,and the beamwidth expands, as the frequency range is enlarged towardlower frequencies.

Discussion of the prior art Although directional microphones haveassumed a number of configurations in the art, one effective arrangementemploys a microphone element placed at the focus of a reflector. Byusing a relatively large reflector, the beamwidth of the system may bemade arbitrarily narrow at a given frequency in order to concentratesound reaching the system at its focus. Thus, the surface of thereflector is shaped so that all of the various pencils of incident soundare concentrated at the focus, i.e., the reflector has a parabolicshape.

Unfortunately, the directional characteristic of such an arrangement isa function of frequency. For a microphone element located at the focusof a large reflector system, a beamwidth of satisfactorily narrowproportions is achieved for high frequencies but is considerablybroadened for lower frequencies. If a fairly broad-band signal, such asspeech, is to be picked up by the microphone, such a variation ofbeamwidth with frequency is undesirable. High frequency signals within arelatively narrow beam are directionally received, but low-frequencysignals from a much broader beam are also received.

Patented Mar. 24, 1970 Conversely, low frequencies from a sound sourcesomewhat off the axis of the reflector will be received fairly well buthigh-frequency components will be lost. As a result, the uniquedirectional properties of the system are lost for a broad-band signal.

The beamwidth of the directional microphone system is, furthermore,dependent upon diffraction effects and is defined precisely only inperfectly focused systems, i.e., in systems in which the microphoneelement is at the focus of the reflection. The beam can be broadened toaccommodate a different range of frequencies by defocusing the system.This is generally accomplished by moving the microphone alongthe axis of'the reflector away from the focal point. The farther the microphone ismoved from the focal point, the broader the beam be comes. Moreover, theamount of beam broadening that can be accomplished by defocusingincreases with increasing frequency. Defocusing, although effective tobroaden the beamwidth for a wide range of frequencies, neverthelessreduces the over-all effectiveness of the microphone and is, at most, acompromise measure.

SUMMARY OF THE INVENTION It is thus a principal object of this inventionto overcome the frequency dependence of prior art directional microphonearrangements. It is another object to achieve a highly directionalmicrophone characteristic that is essentially independent of frequency.

According to the invention, these and other objects are attained bydeveloping signals from energy reaching locations along the axis ofsymmetry of a reflector system as a function of frequency and combiningall of the signals to form a composite output. Since defocusing broadensthe width of the acceptance beam of such a directional system,distributing the points of reception of energy according to frequencyassures a proper degree of defocusing for signals of differingfrequencies to yield a substantially constant beamwidth for all signalfrequencies.

In one embodiment of the invention a parabolic reflector system isequipped with a separate transducer element for each of several discretefrequency ranges established for the system. The transducer element forsignals in the lowest frequency range is placed at the focus of thereflector, and the signals developed by it are passed through a bandpassfilter with frequency limits defined for a specified beamwidth. A secondtransducer is positioned substantially on the axis of the reflector at apoint between the focus and the reflector, and signals derived from thesecond transducer are supplied to a second bandpass filter whosefrequency limits are selected to assure an equivalent beamwidth forsignals of that frequency. Thus, the inherent focusing of higherfrequency signals is compensated by sufficient defocusing to maintain aspecified beamwidth. Similarly, a separate transducer is employed foreach band of frequencies selected for the system, each transducer beingplaced between the focus and the reflector at a point selected to yieldsubstantially uniform beamwidth.

In accordance with another embodiment of the invention, a couplingsystem is employed for recovering energy as a function of frequencyalong the axis of a reflector. The coupling system, typically a cavitywith a tapered port, acts much as a waveguide with a distributedfrequency characteristic, that is, as a low pass filter withcontinuously decreasing cut-off frequency along its length. Bysupporting the coupler in substantial alignment with the axis ofsymmetry of a reflector, received energy is developed at defocusedpoints along the axis to maintain a uniform beamwidth as a function offrequency. Energy recovered by the coupler is supplied to a transducer.

Similarly, any form of distributed transducer system that developssignals of different frequencies along its length may be employed in thepractice of the invention.

DRAWINGS DETAILED DESCRIPTION FIG. 1 illustrates a suitable arrangementfor achieving a highly directional microphone characteristic that isessentially independent of frequency. A plurality of individualtransducers, such as microphone elements 10 10 10 are supported axiallywith relation to a reflector system 11. Preferably reflector 11 is asection of a paraboloid whose focus is at F. Each microphone supplies asignal individually to one of bandpass filters 12 12 12, Filters 12 areselected to divide the frequency band to be accommodated by the systeminto a number of subbands, each one of which is respectively associatedwith one of microphones 10. Signals passed by filters 12 are supplied toan algebraic combining network, e.g., adder 13, to form a compositesignal which is delivered to output terminal 14.

In a system of the type illustrated in FIG. 1, the half power beamwidthof a microphone placed at the focus F of reflector 11 is given roughlyby the formula:

where d is the diameter of parabolic reflector 11 and A is thewavelength of sound being received when measured in the same units as d.Consider, for example, a reflector 6 feet in diameter. Beamwidth for aperfectly focused system as a function of frequency will beapproximately as shown in the table below.

Beamwidth f in Hz.: in degrees 300 37 As the system is defocused, as bymoving a microphone element nearer to the reflector, the beam becomesbroader.

In accordance with the invention, these considerations are turned toaccount, in one embodiment, by placing the microphone corresponding thelowest band, e.g., at the focus F of reflector 11. Signals developed bymicrophone 10 are accordingly passed through filter 12 and restricted toa relatively narrow band of frequencies at the low end of the systemresponse band. The microphone corresponding to the next higher band,e.g., 10 is placed between the focus and the surface of the reflector ata position such that the beamwidth for signals received by microphone 10is the same as that for signals received by microphone 10 at the focus.Signals from microphone 10 are accordingly delivered to bandpass filter12 and limited to the next higher contiguous band of frequencies. Theremaining microphone elements, 10 10 are similarly placed at pointsalong the axis of reflector 11 to assure equal beamwidths forcorresponding frequency bands.

Because of diffraction effects, beamwidth still may vary somewhat withfrequency across each subband. Howeve the amount of variation can bemade as small as is desired by making the bands sufliciently narrow. Inthe lowest band the beamwidth is determined entirely by diffraction andvaries inversely with frequency. In the next band, however, defocusingas well as diffraction contribute to bamwidth determination. Defocusingis approximately independent of frequency and beamwidth is dueessentially to diffraction; hence, it varies somewhat with frequency.This means that the variation with frequency is not so rapid and that abroader subband can be used. At higher frequencies, the contribution ofdiffraction to beamwidth is even less important and the bands can bemade even broader.

In a typical microphone system, a variation of beamwidth of any onetransducer by a factor of two is considered acceptable. This means thatthe lowest band is made, for example, one octave wide. For simplicity,the frquencies corresponding to the edges of this band are designated 1Hz. and 2 7 Hz. The corresponding beamwidths for signals at thesefrequencies are designated 1.0 radian and 0.5 radian. It is desirablethat the beamwidth for the next higher band also varies from 1.0 radianto 0.5 radian. At the lower edge of the band, beamwidth due todefraction is 0.5 radian, so that an additional 0.5 radian of beamwidthmust be obtained by defocusing. As frequency is increased above 2 1 Hz.,the 0.5 radian beamwidth due to defraction decreases, but the 0.5 radianbeamwidth due to defocusin-g remains constant. Thus, all frequenciesabove 2 7 Hz. can be included in one frequency band.

A typical two-microphone system of this construction divides thefrequency band into two parts. Assuming that the frequencies to beaccommodated by the system are all above 600 Hz., the first range isestablished betwen 600 Hz. and 1200 Hz. In the example of FIG. 1, thisrange is established by one of the bandpass filters, e.g., 12 Allfrequencies above 1200 Hz. are accommodated in the second channel; thisrange is established by filter 12 For a reflector 11 with a diameter ofapproximately 12 feet, the beamwidth for such a system varies fromapproximately 9 degrees to 4.5 degrees.

If a smaller variation of beamwidth with frequency is desired, morebands are employed, e.g., additional microphone elements andcorresponding bandpass filters are used. For example, if the beamwidthis to be held within a range of 1 to 1.4, four bands are employed asfollows Lowest band-1 f to 1.4 7 Hz.

Second band1.4 f to 2.4 1 Hz.

Third band2.4 f to 8.4 1 Hz.

Fourth band-8.4 f to upper limit of system response.

In a typical system of this sort in which frequencies above 600 Hg. areto be accommodated, the appropriate bandwidths are as follows:

Lowest band--6'00-840 HZ.

Second band840-1440 Hz.

Third band1440'-5040 Hz.

Fourth band-5040 Hz. to upper limit of system response.

FIG. 2 illustrates the angular defocusing produced by a givendisplacement of a microphone 10 from the focus of reflector 11. Thepencil beam of energy reaching a point P on the surface of thereflector, selected to divide the area of the reflector into two equalparts, from a line parallel to the axis of the reflector, is directed tofocus F of the system. A signal ray from a broader beam, e.g., A0degrees from a line parallel to the axis of the reflector, reaches pointP at a greater angle and hence is directed to point d on the axis of thereflector nearer the reflector surface. Consequently, a microphonelocated at point d, spaced away from focus F, is responsive to signalsfrom a beamwidth of 2M degrees greater than the beamwidth given by theformula That is, the defocused beamwidth is approximately It must, ofcourse, be recognized that the formula which relates broadening of thebeam to axial defocusing is not exact. It is, therefore, best toapproximate the positions of the several transducer elements in a newsystem according to the formula, and, finally, to determineexperimentally the exact microphone positions which produce the desiredamount of beam broadening. Moreover, the system achieves the bestresults when the diameter of the dish is large compared to the longestwavelength of interest. With this relationship, beamwidth is restrictedto a few degrees.

FIG. 3 illustrates another suitable technique. for recovering signalsfrom points along the axis of a reflector system as a function offrequency. In this embodiment, a distributed port coupler 30, or thelike, is supported (by means not illustrated) substantially in alignmentwith the axis of symmetry of reflector 11. Coupler 30 is equipped with atapered port or slot 33. The largest opening of port 33 is made tocoincide with the focus of the reflector system and progressivelynarrower openings are thus placed between the focus and reflector 11.Consequently, coupler 30 acts as a low pass filter with a continuouslydecreasing cut-off frequency. Energy at the lowest frequency arereceived at the opening coincident with focus F, and energy withprogressively higher cutoif frequencies are received nearer thereflector surface. By suitably proportioning port 33, a desiredcharacteristic is achieved for received energy such that uniformbeamwidth across the band is maintained. Transducer 31, associated withcoupler 30, supplies an output signal by way of terminal 32. Techniquesfor constructing suitable couplers are well known to those skilled inthe art and follow, in large measure, comparable techniques used in thefield of microwaves.

It is evident that a variety of techniques may be employed for receivingindividual signals from selected points along the axis of the reflectorsystem. Thus, although a distributed transducer system or individualtransducers are conveniently employed, a single transducer of aline-form with individual segments of response may also be employed.Alternatively, an electrostatic transducer with a common electrode butindividually responsive segments is satisfactory. A tubular transducerwith frequency responsive elements axially spaced may also be employed.Furthermore, additional equalization and amplification elements may beemployed in the several signal channels, if desired, to develop acomposite signal with any desired frequency characteristic, and anydesired form of algebraic combining means may be used.

Numerous other variations may therefore be made by those skilled in theart without departing from the spirit and the scope of the invention.

What is claimed is:

1. A device of the character described which comprises,

reflector means having an axis of symmetry, and

means in substantial alignment with said axis of symmetry for developingsignals representative of energy received at locations along said axisas a function of frequency.

2. A device as defined in claim 1 wherein said reflector meanscomprises,

a paraboloid.

3. A device as defined in claim 1 wherein, said means for developingsignals comprises,

a plurality of transducers,

"means for supporting said plurality of transducers in spaced relationalong said axis of symmetry, and means for combining selected frequencycomponents derived from each of said transducers.

4. A device as defined in claim 1 wherein,

said means for developing signals comprises,

distributed coupling means responsive along its length to signals as afunction of frequency, and

transducer means associated With said coupler for developing signals inresponse to energy received by said coupler.

5. A device of the character described which comprises,

reflector means having an axis of symmetry,

distributed tra-ndsucer means,

means for supporting said transducer means in substantial alignment withsaid axis of symmetry, and

means for combining select frequency components derived from saiddistributed transducer means.

6. In combination,

a reflector,

a trandsucer system responsive to sound energy as a function offrequency along its length,

means for positioning said transducer system between said reflector andits focus along a line substantially congruent with the axis of saidreflector,

means for selecting signals developed by said transducer system toassure substantially the same acceptance beamwidth for all frequencieswithin a prescribed band of frequencies, and

means for combining all of said selected signals.

7. In combination,

a parabolic reflector,

a plurality of acoustic transducers,

means for deriving signals from each of said transducers withinprescribed frequency bands contiguous to one another Within a specifiedfrequency range,

means for positioning individual ones of said transducers atselectively-spaced intervals between said reflector and its focus alonga line substantially congruent with the focal axis of said reflector,

said intervals being selected to establish substantially the sameacceptance beamwidth for each of said transducers regardless of thefrequency band prescribed for said transducers, and

means for combining signals developed by all of said transducers.

8. A directional microphone, which comprises,

a concave reflector,

a plurality of transducers responsive to sound energy for developingsignals within contiguous frequency ranges in the passband of saidenergy,

at least one of said transducers being positioned by the focus of saidreflector and the others of said transducers being positioned at spacedintervals along the axis of said reflector between said reflector andits focus,

said intervals being selected to establish substantially the sameacceptance beamwidth for all of said frequency selective trandsucers,and

means for combining signals developed by all of said transducers.

9. A directional microphone as defined in claim 8,

wherein,

said reflector is a sector of parabola of revolution.

10. A directional microphone as defined in claim 8,

wherein,

said plurality of transducers comprises two elements, the first beingresponsive to acoustic energy in the range of 600 Hz. to 1200 Hz., andthe second being responsive to acoustic energy above 1200 Hz.

11. A directional microphone as defined in claim 8,

wherein,

said plurality of transducers comprises four elements responsive,respectively, to acoustic energy in the ranges of 600 Hz. to 840 Hz.,840 Hz. to 1440 Hz., 1440 Hz. to 5040 Hz., and the ranges above 5040 Hz.

12. A directional microphone as defined in claim 8,

wherein,

said means for combining signals developed by said transducerscomprises,

an algebraic network for l inearly adding together said WILLIAM C.COOPER, Prir nary Examiner developed signals. DOUGLAS W. OLMS, AssistantExaminer References Cited CL UNITED STATES PATENTS 5 34 3 1,897,2222/1933 Weinberger.

